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A parent isotope decays by emitting particles that contain protons, neutrons and electrons. This loss of particles, known as radioactivity, transforms a parent isotope into daughter isotopes. The parent isotope uranium-238, for example, has 92 protons and 146 neutrons. When uranium-238 loses two protons and two neutrons through radioactivity, it becomes the daughter isotope thorium-234.

Because radioactive isotopes decay at a constant rate, the relative proportions of parent and daughter isotopes in a substance help to determine the age of that substance. Scientists have used the relative amounts of a parent isotope, uranium, and a daughter isotope, lead, to estimate the ages of the planets.